Hydor Theo Heater 100 Watt

The Hydor Theo is a high-resistance submersible heater that precisely and safely maintains water temperature in aquariums and turtle tanks.

High resistance submersible internal heater for aquariums and turtle tanks.

The THEO heater has all the essential features for the precise and safe maintenance of the water temperature in your aquarium. The exclusive technology used and the high quality of the components guarantee a long life. Ideal also for turtle tanks, the heater is submersible, anti-thermal shock, highly resistant and features a graduated scale for precise temperature regulation. It can be positioned horizontally or vertically.

Size : 3 x 22.6 cm

CHARACTERISTICS

Compatibility

Tanks. Freshwater and turtle tanks — Hydor build the THEO for aquariums and for turtle tanks specifically. It mounts horizontally or vertically, fully submerged either way, so it fits shallow turtle set-ups as well as deep tanks.

How big a tank this wattage actually heats. Hydor publish two figures, and the second one is the one people miss. ΔT is the gap between the water temperature you want and the room the tank stands in:

• ΔT up to 8 °C: 50 to 100 litres
• ΔT up to 12 °C: up to 75 litres

So a 100 W heater covers a smaller tank in a cold room than in a warm one. If the room drops to 15 °C on a winter night and the tank is held at 27 °C, that is a 12 °C gap and the lower figure is the one that applies. Undersizing is the usual reason a heater runs constantly and never reaches temperature.

Placement. Hydor require it to be fixed with the supplied suction cups somewhere with good water circulation — near a filter outlet or a powerhead. Still water around a heater gives you a warm corner and a cold tank. The bottom end must always be fully submerged and must sit lower than the thermostat at the top end.

Electrical supply. This is the 220–240 V, 50/60 Hz version, which suits Indian mains. Hydor also build a 115 V version for the USA and a 100 V version for Japan — check the label on the heater against your supply, as Hydor instruct.

Physical size: Ø 3 × 22.6 cm. Measure the depth you have before ordering; a 22.6 cm tube will not mount vertically in a shallow tank.

Safety & Precautions

These are Hydor's own warnings from the THEO manual, not general advice.

Before you touch anything in the tank: unplug every appliance in the aquarium before installing or servicing the heater.

Electrical. Check that the voltage on the heater's own label matches your supply. Hydor require the house to be protected by a residual current device — an RCD or ELCB — upstream. Route the cable so its lowest point hangs below the socket, so water running down the cable drips off instead of running into the socket. Indoor use only.

Never switch it on out of water. Do not connect the heater to the mains unless it is submerged. After putting it in, wait a few minutes before switching on so the thermostat can settle to the water temperature.

Thermal shock. Disconnect the heater at least 5 minutes before taking it out of the water, and before a water change or a top-up. Glass at 26 °C meeting cold tap water is how heaters crack. The THEO's tube is double thickness and the PTC element self-limits, but 5 minutes costs nothing.

Water level. The water must always stay above the 'Minimum water level' mark on the heater. Maximum immersion depth is 1 metre.

Damage means scrap, not repair. The mains cable is Z-type wiring and cannot be replaced. If the cable or the heater is damaged the appliance must be discarded — do not use it and do not attempt a repair.

Children. Keep the appliance and its cable out of reach of children under 8. It is not intended for use by people with reduced physical, sensory or mental capability, or without the relevant experience, unless supervised or instructed by someone responsible for their safety. Children must not clean or maintain it unsupervised.

Maintenance. The only routine job is keeping the glass tube clean. Salt or limescale on the tube insulates the thermostat from the water and the heater stops regulating properly. Unplug it before descaling.

What's in the Box

• One THEO heater
• Suction cups for mounting it
• Multi-language user manual

The suction cups are not optional extras — Hydor's manual makes fixing the heater with them a condition of the guarantee, because a heater that works loose and floats can expose its lower end to air and destroy itself. Hydor sell replacement THEO suction cups and clips as a spare, so perished cups do not mean a new heater.

Returns & Replacement

Returns are accepted within 7 days of delivery, provided the item is unused and in its original packaging. Manufacturer-sealed items cannot be returned once the seal is broken, and sale-priced items are not refundable.

If your item arrives defective or damaged, message us on WhatsApp with your order number — or email us, if you would rather — and we will replace it. Approved replacements are delivered within 3 to 5 business days. Transit damage is covered by insurance on every shipment. If tracking shows delivered but nothing reached you, tell us within 72 hours, which is the reporting window Indian couriers allow. Damage caused during installation or use is not covered; replacement spares for such cases are chargeable.

Full terms are on our Cancellation & Returns page.

Warranty & Support

This product is covered by warranty as per the manufacturer policy, against material and manufacturing defects from the date of purchase.

Warranty support is handled by us directly. Message our WhatsApp support with your order number and we will diagnose the fault first — most cases are solved by replacing a single part, so you rarely need to send the whole unit. If the item does need to come back, it ships to our Delhi or Pune warehouse, whichever one served your order, and courier cost in both directions is borne by the customer. Your GST invoice is valid proof of purchase for the claim.